Durga is an incarnation of Devi or the Mother Goddess, a unified symbol of all divine forces. For Shaivas Durga is the wife of Shiva. For Vaishnavas and Shaktas Durga is another form of Uma or Parvati.
The Hindu Goddess Durga manifested when evil forces threathened the very existance of the Gods. To destroy these demons, all gods offered their radiance to her creation and each formed part of Durga's body. Durga also obtained very powerful weapons, such as the chakra from Vishnu and a trident from Shiva.
Durga killed the powerful demon Mahish and all his great commanders. Demonic forces are self-destructive but very powerful. Divine forces are constructive but slow and efficient. When demonic forces create imbalance, all gods unite, becoming one divine force called Shakti or Durga.